This project has the long range goal of immunoprevention of spontaneous neoplasms in animal model systems that have viral and epidemiological similarities to human neoplasia. To this end, past efforts have included a definition of the endogenous type C viruses and spontaneous tumor expressions in large holding colonies of untreated aging mice and more recently the project has developed a number of immunoassays to measure the host's natural and induced immune responses to type C viruses and tumor cells, including cellular and humoral cytotoxicity, footpad, lymphocyte transformation, and radio-immunoassays. This project has found that non-infectious viral and virus containing cellular vaccines can be produced which evoke measurable immune responses in the animal and also result in resistance and protection of the animal to challenge with high doses of leukemogenic virus and tumor cells. Studies of the animal's immune response potential versus aging and his endogenous ecotropic and xenotropic type C virus expression (in his tumor cells and his immune system) will provide information to reach the stated goals. The necessary immunoassay and viral assay techniques are now in hand to sort out this complex issue in natural tumor systems and establish the feasibility of these future studies and achievement of the long range goals.